SINTERING OF FERROMAGNETIC MATERIALS AT LOWER TEMPERATURES IN HYDROGEN
I. Sm₂Co₁₇ ALLOYS

Abstract

Scanning electron microscopy and X-ray energy dispersive spectroscopy were employed to study the sintering of powders from the industrial ferromagnetic Sm₂(Co,Fe,Zr,Cu)₁₇ alloy, melted by the induction method, by the hydrogenation, disproportionation (HD) , desorption, recombination (DR) (HDDR) route. The HD was performed at 700 °C and DR at 950 °C. It was experimentally shown that sintering of the powders occurred already at the HD stage to form a mechanically integral highly porous material. The porosity of the sintered materials was found to decrease when the compaction pressure and powder particle dispersion increased. The powder compaction pressure was estimated and established to range from 2 to 5 t/cm². The decrease in the sintering temperature was explained by increase in the diffusion rate of the alloy components resulting from decrease in the particle size, hydrogen-initiated phase transformations, the hydrogen solid solution present in the alloy. The phase transformations occurred when the pressure changed at the high temperature. If the hydrogen pressure was high, the intermetallic was not thermodynamically stable and disintegrated (disproportionated) into several phases. If the hydrogen pressure was low (vacuum), the rare earth metal hydride was thermodynamically unstable and disintegrated, while the rare earth metal interacted with other phases to form the starting intermetallic. These phenomena are chemical reactions in a solid body and proceed through the diffusion of components. The new method of sintering for ferromagnetic materials has process advantages over the existing methods: it does not require holding at the highest heating temperatures or usage of complex molds or complex equipment and results in the production of anisotropic nanostructured materials. Ways to improve the properties of sintered materials at low temperatures (in particular, increasing the homogeneity of their microstructure and decreasing the porosity) are proposed: optimization of sintering parameters and homogenization of the powders by particle size.